1. Field of the Invention
The present invention relates to a method of and an apparatus for detecting cracks in a piezoelectric element that deforms according to a voltage applied thereto.
2. Description of Related Art
Small-sized, precision information devices are rapidly advancing, and for use with such devices, needs for micro-actuators capable of conducting positioning control for very small distances are increasing. The micro-actuators are highly needed by, for example, optical systems for correcting focuses and inclination angles, ink-jet printers for controlling ink heads, and magnetic disk drives for controlling magnetic heads.
The magnetic disk drives increase storage capacity by increasing the number of tracks per inch (TPI), i.e., by narrowing the width of each track on a magnetic disk.
Large-capacity magnetic disk drives, therefore, need an actuator capable of precisely positioning the magnetic head within a minute range across tracks.
To meet the need, Japanese Unexamined Patent Application Publication No. 2002-184140 proposes a head suspension employing a dual actuator system. The dual actuator system uses a piezoelectric actuator in addition to a usual voice coil motor that drives a carriage to which the head suspension is attached. The piezoelectric actuator includes a piezoelectric element and is arranged between a base plate and a load beam of the head suspension.
In addition to moving a magnetic head of the head suspension with the voice coil motor, the head suspension employing the dual actuator system finely moves the magnetic head in a sway direction (a width direction of the head suspension) with the piezoelectric element that deforms in response to a voltage applied thereto, thereby precisely positioning the magnetic head on a magnetic disk in the magnetic disk drive.
The magnetic disk drive is required to be small and thin, and accordingly, the piezoelectric element is required to be thin.
A thin piezoelectric element easily causes microcracks due to external force applied thereto during production or assembling into a head suspension. The piezoelectric element having microcracks deteriorates a long-term reliability, and therefore, must be discarded as a defective.
The microcracks, however, are difficult to find by external observation with a stereomicroscope. Furthermore, the surface of the piezoelectric element is plated with gold to form electrodes, and therefore, finding microcracks by appearance is hardly achievable.
The microcracks are also undetectable by measuring electric characteristics of the piezoelectric element. When assembled in a head suspension, the piezoelectric element is tested and evaluated by measuring the capacitance thereof. The microcracks, however, cause no change in the capacitance of the piezoelectric element.
To deal with this, Japanese Unexamined Patent Application Publications No. H06-003305 and No. 2002-367306 disclose a technique of examining the impedance-frequency pattern or phase-frequency pattern of a piezoelectric element, or testing the optical transmissivity of a piezoelectric element.
These related arts, however, are unable to easily detect microcracks in the piezoelectric element, or are difficult to achieve in practice. Namely, the related arts are unable to surely find out cracks including microcracks in a piezoelectric element in practice. As a result, most related arts concentrate on preventing cracks from being formed in piezoelectric elements.